What is correct in this situation?

You answered correctly. In Germany, you may drive on an advisory bike lane if it is necessary, like when passing an obstacle such as a truck, but only after ensuring that no cyclists are endangered or obstructed. This means you must carefully observe the road to rule out any risk to cyclists, and limit your use of the bike lane to the minimum distance required, such as just while passing the truck. You are not generally prohibited from using the advisory bike lane, but you must always prioritize the safety of cyclists. This rule helps ensure both smooth traffic flow and the protection of vulnerable road users.